High currents in switchgear are carried using thick copper bars (the bus) of varying cross-sections. The main bus of the switchgear runs the entire length of the switchgear. The bus is always coated with an insulating layer (epoxy coating or shrink wrap coating). As the bus extends from one switchgear frame to another, the bus needs to be structurally supported. Additionally, the structural support needs to be such that if there is an internal arc fault in the bus compartment of a frame, the arc gasses do not freely flow into the adjoining compartment. This is a difficult requirement since the arc pressures generated are quite high. Some leakage is permissible into the adjoining compartment but then, there should be zero gas intrusion into the next compartment.
An example of a conventional a bushing structure is shown in FIG. 1, where the bushing structure 10 is mounted to an outside surface of a switchgear frame 12 using a plurality screws 16 engaged with a plurality of Tinnerman nuts 18. Thus, the bushing 10 is sandwiched between the switchgear frames 12 and 14. The bushing structure 10 includes three bushings 20, each having a through-hole 22 for receiving a current carrying bus (not shown). The bushings 20 make it possible to pass the buses through openings 24 of the frames 12 and 14. The bushing structure 10 is made of molded glass epoxy and once assembled, the configuration relies on using putty or sealing material to close the gap between the bus and the bushing 16. Additionally, due to BIL and heat rise considerations, porcelain bushings are required for ratings of 3000 A and above. This multi-piece bushing structure 10 makes assembly difficult and tedious and also makes it difficult to control the quality of the final assembly.
Thus, there is a need for a bushing structure that is of simple configuration, is easy to assembly, and has improved quality.